An information communication network represented by the Internet has been set up worldwide as an infrastructure essential to our daily life. As a technology for supporting traffic of the information communication network, there is an optical communication technology that uses optical fibers. A silicon based optical communication device capable of using a band of 1.3 micrometers and a band of 1.5 micrometers in an optical fiber communication wavelength range is very promising in that using a CMOS (Complementary Metal Oxide Semiconductor) technology enables integration of optical functional elements and electronic circuits on a silicon platform.
As one of methods of dealing with yearly increasing traffic of the information communication network, there is a method of increasing the information transmission speed per channel. To achieve this, an optical modulator that is one among various optical devices for converting a signal from a LSI (Large Scale Integration) circuit responsible for signal processing in the optical communication device into an optical signal at a high speed is important. It is desired that such an optical modulator be realized on a silicon platform.
Representative among optical modulators presented thus far is a modulator that changes optical waveguide characteristics by using a carrier plasma effect to change the refractive index of a silicon material. For example, Nonpatent Literature 1 discusses an optical modulator that uses a p-n (positive-negative) junction that operates when a reverse bias voltage is applied. There is also an optical modulator that uses a MOS (Metal-Oxide-Semiconductor) capacitor. Both optical modulators can operate at high speeds.
FIG. 1 shows an example (refer to Nonpatent Literature 1) of a silicon based optical modulator according to a related art, which is one of optical devices and uses a waveguide formed on a SOI (Silicon on Insulator) substrate.
Oxide layer 25 and p-doped silicon layer 23 that has been doped are sequentially stacked on substrate 24 to form a SOI substrate. On an upper surface of p-doped silicon layer 23 formed into a rib shape, n-doped silicon layer 21 that has been formed into a reverse rib shape and that has been doped is deposited. On one side of n-doped silicon layer 21, n+ doped silicon layer 20 doped with a high concentration is located. On both sides of p-doped silicon layer 23, p+ doped silicon layers 22 doped with high concentrations are formed. Electrode 27 is connected to n+ doped silicon layer 20 and p+ doped silicon layer 22. Oxide layer 25 covers the entire optical modulator. Rib-shaped part 23′ of p-doped silicon layer 23 and reverse rib-shaped part 21′ of n-doped silicon layer 21 constitute a waveguide, and optical modulation is performed in the waveguide by applying a reverse bias voltage to electrode 27.
In the case of the optical modulator that includes the p-n junction or the MOS capacitor, when the optical modulator is connected to an optical waveguide disposed outside the optical modulator, the waveguide of the optical modulator and the waveguide of the optical waveguide are different in structure. Such a sudden shape change of the waveguide at a connection portion of both waveguides different in structure causes reflection of light, generating light coupling losses at the connection portion of the optical waveguide and the optical modulator. The coupling losses may reduce, in addition to an increase of insertion losses of light to the optical modulator, optical modulation efficiency of the optical modulator. This necessitates establishment of a connecting channel between the optical modulator and the optical waveguide to reduce coupling losses.
For example, as shown in FIG. 29 and FIG. 30 of Patent Literature 1, light losses are reduced at the connection portion of the optical waveguide and the optical modulator by forming a single sharp-pointed taper (one of input increase taper, output decrease taper, input decrease taper, and output increase taper) at each of an input part and an output part of each silicon (Si) layer stacked at two stages in an input/output part of the optical modulator.